Space is on the right track I reckon. You should arrange a nice day for her, perhaps take her to a movie that she wants to see, and/or take her for lunch (NOT dinner).
What you're looking to do is take her mind off what is depressing her, and she'll come back to life. Obviously she must make the decision that the rejection doesn't matter after all, but it's a lot easier to do this if it's not the only think that she can think about.
Girls are (no offence Blazing20, Tatinee, Lotus350) relatively easily distracted, so you just need to think of something nice that you can enable her to do, even if you're going to hate it.
But (and I've already said this many a time here on GTP), you have to put your own desires second. If she gets even a sniff that you're interested in anything other than her mental well-being, then the whole plan will backfire.
Remember, there's a chance that she didn't go for you because she was interested in this other guy. If you come across as some horny kid who only wants to get his hand in her top, then you're gonna be in a whole world of pain.
Be nice and sympathetic, and make her a nice day. She'll really appreciate it.